Vehicles designed to navigate in fluid media (i.e., water and air) are often driven by conventional propellers. Such devices typically comprise a rotating shaft having blades mounted thereto. Each blade is mounted such that its longitudinal axis is generally normal to the shaft and its transverse axis forms an oblique angle with the shaft. Continuous rotation of the shaft causes the blades to turn within the fluid; in doing so, the blades generate thrust along the shaft longitudinal axis, with the direction of the thrust (and the direction of travel of the vehicle) being determined by which edge of the blades leads during rotation. Such propellers are very commonly used as the primary propulsive device in airplanes, power boats and submarines.
Propellers are particularly effective in generating a steady state fluid force, which enables propeller-driven vehicles to cruise at a steady speed. However, propellers typically require blade travel of several chord lengths to generate their maximum force for a given rotation rate in order for the steady-state circulation pattern to form around the blades. As a result, propellers are not particularly effective at producing short duration, large impulses, such as those needed to maneuver a vehicle quickly. Also, the direction of thrust production is unchangeable, always coaxial with the propeller shaft, and requires extra machinery for redirecting that shaft or the thrust using a rudder. The inability to maneuver the vehicle can present safety hazards, especially in conditions where there are permanent structures, wildlife, or rapidly changing conditions in the fluid medium.
To improve maneuverability of water and air craft, fluid impellers have been developed. Fluid impellers typically include a blade or foil that is moved through the fluid with a reciprocating or pivoting motion. Most fluid impellers can be categorized as one of three basic types: flexible rudders; sweeping (or flapping) blades; and "heave and pitch" devices. Each of these is described in detail below.
A flexible rudder typically consists of an elongate flexible plate that is often shaped like a fish or fishtail. The plate is mounted to the vehicle via a rigid driveshaft (usually vertical) that is, in turn, mounted in a rigid bushing into the vehicle hull. The driveshaft is pivoted about its longitudinal axis to generate a sweeping motion of the "tail" of the blade, with the pivot axis passing through the blade. Fluid resistance creates a phase lag between the rotary sweep motions of the nose and tail of the blade, which generates and propagates bending waves traveling tailward. The flexible rudder is advantageous because, if properly designed, it can be used to create thrust in any direction normal to the driveshaft. However, the hull of the vehicle will shake and vibrate due to the unbalanced lateral (heaving) and the longitudinal (thrust) forces generated by the waving blade in its fixed mount. Also, the thrust output dips to zero near the natural frequency of the system, as two nodes are created on the flexible blade: one at the driveshaft; and one at approximately three-quarter of the blade length (i.e., a standing wave results), thereby destroying the propagation of waves.
Sweeping or flapping blades employ a rigid driveshaft that is rotatively mounted in a bushing on the hull of the vehicle. The driveshaft then extends in a direction normal to the axis of rotation of the bushing (often horizontal), then extends parallel to the axis of rotation (often vertical). The blade attaches to this latter vertical portion of the driveshaft, either rigidly (in the case of a flexible foil) or with a spring joint (in the case of a rigid foil). Reciprocation of the driveshaft about its pivot point within the bushing generates a sweeping motion of the blade. Fluid resistance creates a phase lag between the leading and trailing edges, which generates thrust. Sweeping blades can be advantageous over flexible rudders in that the larger area of sweep can produce more rapid travel of the trailing edge of blade, which can produce a higher thrust. However, because of its position relative to the craft, it generally cannot produce thrust in all directions. In addition, the craft tends to shake due to the unbalanced forces along or acting along the lateral, thrust, and torsional axes generated by the reciprocating lever arm in its fixed mount.
Heave and pitch devices typically include one or more rigid plates attached at the ends of rigid driveshafts. Typically, each driveshaft is mounted in a rigid coupling within which it can pivot; this coupling is then mounted to a sliding carriage that slides within a track. The track is then mounted to the hull of the craft. In operation, the carriage reciprocates on the track; simultaneously, the blade is actively pivoted in a manner to generate lift, as in a hydrofoil. Compared to flexible rudders and sweeping blades, heave and pitch devices better control the balance between lift and drag. Unfortunately, directional control is typically inferior to that of flexible rudders, and thrust in all directions is not possible without re-orientation of the carriage. Also, these devices are more complicated (i.e., have more moving parts) than flexible rudders and sweeping blades. Moreover, the vehicle hull tends to shake and vibrate due to the oscillatory thrust forces generated by the driveshaft in its longitudinally fixed mount absent some control apparatus designed to eliminate thrust oscillation. Note that the prior art discloses the use of multiple blades working in opposition to attempt to cancel the heave axis vibration inherent to all oscillatory propulsors. However, this requires multiple blades and that such blades always work in opposition. Even so, this still fails to address and correct for other vibrations, such as those along the thrust axis.